A growing need to produce portable and wireless electronics with extended lifespans put constantly increasing strain on available power sources for such systems. Current mobile devices usually must be designed to include electrochemical batteries as the power source. The use of batteries can be often troublesome due to their limited capacity and lifespan, thus necessitating their periodic recharging or replacement.
One of the technologies that holds a promise to substantially alleviate current reliance on the electrochemical batteries is energy harvesting. Energy harvesting devices are designed to capture the ambient energy surrounding the electronics and convert it into usable electrical energy. The concept of energy harvesting works towards developing self-powered devices that do not require replaceable power supplies.
Many types of energy harvesters exist, each offering differing degrees of usefulness depending on the application. Perhaps the best-known energy harvesters are solar cells, which have long been used to power simple hardware components such as calculators or emergency telephones.
Another type of harvesters converts the energy contained in a vibrating object into electrical energy. These systems have been demonstrated to extract energy from floors, stairs, and equipment housings.
A third type of harvesters uses mechanical energy, such as that produced by a person walking and an object's movement. For example, some electronic watches, currently commercially available, operate by converting mechanical energy available from the swing of a person's arm to useful electrical power.
Currently the power output of energy harvesters is substantially limited by the efficiency of the energy converting transducers and the raw energy available, remaining in the microwatt to milliwatt range. However, recently, with the advent of mobile computing, the demand for more powerful energy harvesting devices with the output on the order of watts or even tens of watts has substantially increased.
In that respect, harvesters that convert mechanical energy into electrical energy are particularly promising as they can tap into high power sources such as human motion. For instance, from resting to a fast sprint, the human body expends roughly 0.1 to 1.5 kilowatt. Only part of this energy is available for harvesting, but even a modest part of this vast energy pool can constitute a substantial power source.
For instance, one of the promising ways to extract energy from people's motion is by tapping their gait. Humans typically exert up to 130 percent of their weight across their shoes at heel strike and toe-off, and standard jogging sneakers cushioned soles can compress by up to a centimeter during a normal walk. For a 154-pound person, this indicates that about 7 Watt of power could be available per foot at a 1-Hertz stride from heel strike alone. For comparison, such relatively power-hungry mobile electronic devices as mobile phones and laptops typically consume power on the order of 1 Watt and 15 Watt respectively.
Successful high-power mechanical energy harvesting requires an efficient transduction mechanism to generate electrical energy from environmental mechanical motion. One of the key requirements is to maximize the coupling between the mechanical energy source and the transduction mechanism. This is a complicated problem due to often unpredictable aperiodic nature of environmental mechanical motion and a very broad range of forces, displacements and accelerations, exhibited by this motion.
Currently there are three major types of mechanical-to-electrical energy converting devices, or transducers as they are sometimes called, namely piezoelectric, electromagnetic, and electrostatic. Each of them has their respective advantages and shortcomings, but none of them can currently provide a high-power-output solution capable of effectively coupling to a broad range of environmental mechanical motion.
In particular, piezoelectric transducers are inexpensive, lightweight, compact, have no moving parts, and can be easily incorporated in a broad range of devices. However, they require high-stress, low-displacement mechanical motion, generate high-voltage, low-current output, and have low conversion efficiency.
To the contrary, electromagnetic systems are best adapted to high-displacement, low-force mechanical motion. Their conversion efficiency can be high and the output voltage can be tuned to a broad range of values. Unfortunately, due to a high-displacement requirement they typically need additional complicated mechanical or hydraulic mechanisms to achieve effective coupling with the common types of environmental mechanical motion. This often makes such systems prohibitively bulky and expensive.
Electrostatic systems are typically inexpensive, lightweight, have high conversion efficiency and potentially a broad range of tunablty in terms of allowable mechanical force and displacement. However, in currently employed systems high values of electrical field are typically required to achieve even modest levels of output power. These high values of electrical field translate into high operational and output voltages, making these electrostatic devices impractical in many applications.
Thus, there is clearly a need for a simple, compact, and efficient high-power mechanical-to-electrical energy converting device capable of combining a wide range of possible output voltages with the ability to effectively couple to environmental motion with a broad range of mechanical displacements and forces.